In conventional LTE (Long-term evolution) systems, which are one kind of radio communication system, common reference signals (hereafter, CRS) are used to carry out channel estimation, coherent detection, cell search and so forth, in the downlink. In LTE systems, data and so forth can be transmitted using a plurality of transmission antennas (MIMO: Multi-Input Multi-Output). Transmission is performed using at most four transmission antennas.
LTE-A (LTE-Advance) systems are another kind of radio communication system. In LTE-A systems, like in LTE systems, channel estimation and so forth is performed using CRS.
FIG. 25 is a diagram illustrating a configuration example of a subframe. FIG. 25 illustrates an example of transmission of CRS using four transmission antennas, with time on the abscissa axis and frequency on the ordinate axis. The base station transmits for instance CRS and data, from each antenna, using scheduled subframes. The terminals as well hold subframes such as those illustrated in FIG. 25, and receive the CRS, data and so forth from the base station at each time and each frequency.
FIG. 26 is a diagram illustrating an example of channel resource allocation. For instance, one resource block corresponds to the subframe of FIG. 25. In some cases, the base station transmits CRS, data and so forth in a situation where LTE compliant terminals are mixed with LTE-A compliant terminals. In such a case, the base station transmits CRS, data and so forth using two kinds of resource blocks (resource blocks for LTE compliant terminals and resource blocks for LTE-A compliant terminals), as illustrated in FIG. 26.    Non-patent Document 1: 3GPP TS 36.211    Non-patent Document 2: 3GPP TS 36.212    Non-patent Document 3: 3GPP TS 36.213    Non-patent Document 4: 3GPP TR 36.913 v8.0.0    Non-patent Document 5: 3GPP TSG RAN1. #55, R1-084251
In LTE and LTE-A, as described above, the base station allocates resources, for CRS transmission, to at most four transmission antennas (see FIG. 25).
In a case where the base station uses five or more transmission antennas, however, the way in which reference signal resources are to be allocated for the respective five or more transmission antennas has not been established yet.